The joining of metal parts such as formed shapes, forgings, castings, or plates to construct any number of structures or components for a variety of industries is largely performed by fusion welding. For example, construction using pipes and tubes to form pipelines for oil, gas and geothermal wells and the like is largely performed by conventional arc or fusion welding. For decades, the pipeline industry has made wide use of several fusion welding technologies such as shielded metal arc welding (SMAW) and mechanized gas metal arc welding (GMAW) for pipeline construction.
Arc or fusion welding involves melting of the materials being welded to create the joint. In such a process the larger the pipe diameter, or the thicker the wall of the pipe, the slower the welding becomes because a greater volume of metal must be melted and deposited in the weld joint. For onshore pipelines, particularly in remote areas, it is important that the welding be as economical as possible because of the large expense related to deployment of workers and equipment to the pipeline right of way (ROW). For offshore pipelines, it is important that the welding be as economical as possible because of the substantial costs associated with the laybarge. Several other factors such as availability of qualified welders, pipe thickness, welding productivity, quality of weld and automation of the welding process etc. may also play a role in the selection of welding process.
Girth welds of linepipe steels using the well established fusion welding processes typically consists of 3-20 passes of weld beads depending on the thickness of the pipe. Pipelines are built by girth welding individual joints of pipe together. During a standard pipeline construction process, mainline welding is accomplished by having about as many welding stations as there are number of weld passes, each station designed to produce one or two specific weld passes. The entire process, therefore, requires considerable manpower and associated expenses to house and support the workers, particularly in remote locations. The construction process is also time consuming, which impacts pipeline construction costs. Sometimes two pipes are first welded in a shop to create a “double joint” and then the double joints are transported to the pipeline right of way (ROW) for final field construction. In a typical field construction process, the pipe ends are butted together and then a welding process is used to fuse together the butted surfaces. Filler metal is also added to the weld as it is being made. Welding shacks are often used at each welding station to protect the external welding activity from the weather elements. Typically, each shack is devoted to applying one or two welding beads.
Friction Stir Welding (FSW) is a solid state joining technique that uses a rotating (spinning) tool to stir metal together to form a joint or weld. The spinning tool is pressed into the material being welding with considerable force. A downward force is applied perpendicular to the pieces being welded. A translational force(s) is applied roughly parallel to the surfaces of the pieces being welded, this force being used to translate the tool along the weld joint. If the weld joint consists of a curved path, there may also be transverse reaction forces to be supported by the FSW equipment. FSW is suitable for butt welding, but may be applied to other joint configurations. The heat generated during FSW softens the material adjacent to the tool and reduces its strength. This softening (which extends a few millimeters from the tool) is necessary to plasticize the material and allow it to be stirred. The softened area that becomes mechanically mixed is commonly referred to as the stir zone. Because the material in the stir zone is heated to a relatively high temperature during friction stir welding, it is weakened. This also includes the material in the root area, which is the area of material to be welded that is just below the tip of the spinning tool. Hence, during FSW, the root area is typically supported from beneath the work pieces such that the stir zone does not displace away the bottom of the weld joint. A backing support plate is often used to provide support to the stir zone during FSW. In the absence of a backing support to resist the down forces exerted during FSW, the material in the stir zone of the weld may displace away from the bottom of the weld joint resulting in a defective weld or joint.
It is desired to use FSW to join metallic components without the use of an independent backing support and to solve the problem of common root defects that can occur with the use of a backing support. More specifically, it is desired to use FSW without a backing support to construct oil and gas pipelines. FSW can join in a single pass the entire wall thickness, or nearly the entire wall thickness, of pipes which would require multiple arc welding passes. However, one of the problems in applying FSW for pipeline construction relates to the need for backing support and how to accommodate any practical means of backing support into the considerable “front end” activity that takes place when pipes are welded onto a newly constructed pipe string. At the front end of the construction process, workers are conducting such activities as pipe beveling and preheating, which may limit access to the inside of the pipe for use of an internal backing support to counter the FSW down force. Such an internal backing support would also be a bulky piece of equipment requiring some means of power and remote control, which would be difficult and costly. Additionally, the use of a backing support increases the likelihood of root defects because backing supports cannot easily accommodate the common problems of pipe misalignment, variations in pipe diameter, and variations in wall thickness. The use of a backing support for FSW during pipeline construction is problematic.
Hence there is a need for a butt weld and method of forming the butt weld using FSW that may be formed with a sufficiently strong root weld area such as to avoid the need for a backing support during FSW, whereby said butt weld and methods to form butt welds accomplish the goal of minimizing root defects in light of common geometric misalignments that occur in root areas when two workpieces are butted together.